Margaret A. Sparrow

TERRESTRIAL ORGANIC CARBON CONTRIBUTIONS TO SEDIMENTS ON THE WASHINGTON MARGIN

Elemental and stable carbon isotopic compositions and biomarker concentrations were determined in sediments from the Columbia River basin and the Washington margin in order to evaluate geochemical approaches for quantifying terrestrial organic matter in marine sediments. The biomarkers include: an homologous series of long-chain n-alkanes derived from the surface waxes of higher plants; phenolic and hydroxyalkanoic compounds produced by CuO oxidation of two major vascular plant biopolymers, lignin and cutin. All marine sediments, including samples collected from the most remote sites in Cascadia Basin, showed organic geochemical evidence for the presence of terrestrial organic carbon. Using endmember values for the various biomarkers determined empirically by two independent means, we estimate that the terrestrial contribution to the Washington margin is ~ 60% for shelf sediments, ~ 30% for slope sediments, and decreases further to ≤15% in basin sediments. Results from the same geochemical measurements made with depth in gravity core 6705-7 from Cascadia Seachannel suggest that our approach to assess terrestrial organic carbon contributions to contemporary deposits on the Washington margin can be applied to the study of sediments depositing in this region since the last glacial period.

A biomarker perspective on prymnesiophyte productivity in the northeast pacific ocean

Abstract Long-chain alkenones derived from prymnesiophyte algae were analysed in 1-year sediment trap time series (September 1987–1988) from three sites along a 630 km offshore transect at ∼42°N in the northeast Pacific Ocean. Biomarker flux monitored at 1000 m water depth was evident throughout the year at all sites and showed a consistent seasonal maximum in late spring which increased in amplitude with distance offshore. The integrated annual biomarker flux was constant along the transect, despite differences in seasonality between sites. Alkenone unsaturation patterns were remarkably uniform throughout the time series, reflecting an algal growth temperature of 10.6 ± 1.1°C. This value corresponds to regional water temterature at the sea-surface in winter. It recurs in seasonal upwelling near the coast and at the depth of the subsurface chlorophyll maximum offshore during seasons of stratification. These biomaker observations, interpreted in view of trap data for total organic (TOC) and inorganic carbon and ancillary hydrographic information, help to clarify seasonal productivity patterns for alkenone-producing prymnesiophytes in the northeast Pacific Ocean. Sediments accumulating with distance offshore along the sampling transect change from suboxic and Mn-reducing at the water-sediment interface to aerobic throughout the depths penetrated by box coring. Comparison of alkenone and TOC accumulation rates in surface (0–1 cm) sediments with corresponding annual fluxes integrated by the trap time series, shows that the fraction of both properties accounted at the seafloor is highest and similar under sub-oxic conditions (∼25%), and declines steeply and disproportionately as aerobic conditions are encountered farther offshore. Only 0.25 and 3.1% of the annual inventory for alkenones and TOC in traps are accountable in surface sediments from the slowest accumulating, most oxidizing site farthest offshore. Despite major loss of biomarker to early diagenesis, surface sediments and trap particles display consistent alkenone unsaturation patterns. Results from this study provide a necessary background for palaeoceanographic reconstruction of the northeast Pacific Ocean from stratigraphic analysis of alkenone abundances, unsaturation patterns and isotopic compositions in sediment cores.

Physiological impacts on alkenone paleothermometry

We conducted isothermal (15°C) batch culture experiments with the coccolithophorid Emiliania huxleyi (strain NEPCC 55a) to evaluate the extent to which nutrient and light stress contribute to variability in the alkenone unsaturation index U37K′. Alkenone content and composition were constant throughout exponential growth in both experiments when nutrients (nitrate and orthophosphate) were replete. Stationary phase (nutrient-starved) cells continued to produce alkenones, amassing concentrations (ΣAlk) ≥ 3 times higher than those dividing exponentially (1.5–2 pg cell−1), and the U37K′ of “excess” alkenone dropped by 0.11 units. In contrast, 5 days of continuous darkness resulted in a 75% decrease in cellular ΣAlk and a significant U37K′ increase (+0.11 units). Given an established 0.034 unit/°C response for exponentially growing cells of this strain, the observed range of U37K′ variability at 15°C corresponds to an uncertainty of ±3.2°C in predicted growth temperature. This level of variability matches that of the global U37K′ annual mean sea surface temperature calibration for surface marine sediments, begging the question: What is the physiological condition of alkenone-producing cells exported to marine sediments? Comparison of our laboratory results for a strain of E. huxleyi isolated from the subarctic Pacific Ocean with depth profiles for alkenones in surface waters from two contrasting sites in the northeast Pacific Ocean suggests that the answer to this question depends on the ocean regime considered, a possibility with significant bearing on how stratigraphic U37K′ records in marine sediments are to be interpreted paleoceanographically.

Assessment of sea-surface temperature at 42°N in the California Current over the last 30,000 years

Assessment of changes in surface ocean conditions, in particular, sea-surface temperature (SST), is essential to understand long-term changes in climate especially in regions where continental climate is strongly influenced by oceanographic processes. To evaluate changes in SST in the northeast Pacific, we have analyzed long-chain alkenones of prymnesiophyte origin at 38 depths in a piston and associated trigger core collected beneath the contemporary core of the California Current System at 42°N, ∼270 km off the coast of Oregon/California. The samples span 30,000 years of deposition at this location. Unsaturation patterns (U37K′) in the alkenone series display a statistically significant difference (p ≪ 0.001) between interglacial (0.44 ± 0.02, n = 11) and glacial (0.29 ± 0.04, n = 20) intervals of the cores. Detailed examination of other compositional features of the C37, C38, C39 alkenone series and a related C36 alkenoate series measured downcore suggests the published U37K′- temperature calibration (U37K′=0.034×T+0.039), defined for cultures of a strain of Emiliania huxleyi isolated from the subarctic Pacific, provides best estimates of winter SST at our study site. This inference is purely statistical and does not imply, however, that the phytoplankton source of these biomarkers is most productive in winter or at the ocean surface. The temperature record for U37K′ implies (1) an ∼4°C shift occurred in winter SST from ∼ 7.5 ± 1.1°C at the last glacial maximum to ∼ 11.7 ± 0.7°C in the present interglacial period, and (2) this warming trend was confined to the time frame 14–10 Ka within the glacial to interglacial transition period. These conclusions are corroborated entirely by results from an independent SST transformation of radiolarian species assemblage data obtained from the same core materials.

Annual biomarker record for export production in the central Arabian Sea

The record for plankton biomarkers in sediment trap samples from a one-year experiment in the central Arabian Sea (AS4: 15°59′N 61°30′E) shows variations that reflect changing biological conditions in surface waters. Particulate fluxes of C37–39 alkenones, highly branched C25 isoprenoids (HBI), dinosterol, nC28 12-hydroxy fatty acid, 24-ethylcholesterol, and a C30–34 series of pentacyclic triterpanols all displayed distinct maxima at the start and stop of the Northeast (NE) and Southwest (SW) Monsoons. Surface mixing conditions changed rapidly at these times, altering light and nutrient availability, thereby triggering these biomarker signals of export production. Temporal offsets noted in individual biomarker concentrations (per g total organic carbon) at the start of the SW Monsoon suggest succession occurs in the phytoplankton community contributing to organic matter export. Comparable offsets were neither apparent at the start of the less dynamic NE Monsoon nor at the end of the NE or SW Monsoons. Broad concentration maxima for HBI also were observed at the beginning and end of the time-series during the relatively quiescent Fall Intermonsoon period when such features were conspicuously absent for other biomarkers. HBI are reputed biomarkers of Rhizoselenia and Haslea spp., two recognized dominants of diatom biomass in the Arabian Sea. These peaks in biomarker concentration could reflect either changes in the relative proportion of specific organisms that contribute to the upper ocean productivity or enhanced preservation of the biomarkers during times of high export production. In either case, the biomarker record in sediment traps reflects important changes in the biological condition of the upper ocean. All biomarkers except HBI were measurable in surface sediments deposited beneath the trap site. Comparison with concentrations in average sediment trap particles showed each was sensitive to significant (∼99%) degradation, displaying depletion factors relative to TOC of ⩾4. Clearly, consequences of such high levels of early diagenetic recycling must be considered carefully when conclusions about changes in export production from surface waters in past oceans are drawn from stratigraphic analysis of biomarkers in marine sediments.

Seasonal record for alkenones in sedimentary particles from the Gulf of Maine

Abstract C37–39 alkenones and C36 alkenoates, biomarkers of haptophyte origin, were measured in a 10-month sediment trap times series from the Wilkinson Basin in the Gulf of Maine (GOM). Highest biomarker flux to the seabed was observed in summertime, the period when surface waters are stratified and a persistent, subsurface chlorophyll maximum (SCM) exists within the upper thermocline and at the base of the euphotic zone throughout the GOM. Comparison of biomarker content and composition of sediment trap particles and underlying surface sediments indicates significant loss (>50%) of signal due to the impact of early diagenesis. Despite such loss, however, C37 alkenone unsaturation patterns (U37K′) are not altered. Estimates of algal growth temperature made from analysis of U37K′ in these sedimentary materials correspond with water temperature measured at the SCM, identifying this biological oceanographic feature as a key site of alkenone export production to the GOM sediment record. Given the common occurrence of SCM in surface waters of the world ocean, particularly the expansive oligotrophic regions of the subtropical to temperate ocean, export of alkenones produced within such features is a potentially widespread biological oceanographic phenomenon which shapes the sediment record for these biomarkers.

Phytane from chemolytic analysis of modern marine sediments: A product of desulfurization or not?

A set of modern marine sediments was analyzed to evaluate the role of sulfurization as a mechanism for biomarker preservation during early diagenesis. The set consisted of sediments accumulating within various oxic to anoxic depositional environments. Raney nickel treatment of the polar fraction of total extractable lipids (pTEL) from each sample yielded 3,7,11,15-tetramethylhexadecane (‘phytane’) as the dominant product, accompanied in several cases by minor levels of C27, C28, and C29 5α/β(H)-steranes, n-C31 alkane, and β-carotane. Although others have ascribed such products to a desulfurization reaction, our work reveals significant production of phytane from the action of Raney nickel on the esterified side chain of chlorophyll. Results suggest that catalytic dehydration/hydrogenation of chlorophyll accounts for ~50 and ~4% of the Raney nickel phytane (rnPhy) yield measured in suboxic sediments from the Washington shelf and anoxic sediments from Saanich Inlet, respectively. If 8–20% of the pheopigment detected in the suboxic sediments and all of the pheopigment detected in the anoxic sediment exists as pheophytin, the catalytic dehydration/hydrogenation process would account for 100 and 50%, respectively, of the observed rnPhy yield. The collective findings from our work underscore the necessity to exercise extreme caution when interpreting the geochemical significance of hydrocarbon products from Raney nickel treatment of pTEL from sediments. Despite the organic chemical fact that Raney nickel is a well-known desulfurizing agent, desulfurization cannot be assigned necessarily as the source of hydrocarbons generated by the action of this reagent on complex lipid extracts from sediments, particularly those from modern environments.

Dissolved organic carbon in interstitial waters from sediments of Middle Valley and Escanaba Trough, Northeast Pacific, ODP Legs 139 and 169

Oxidation measurements of nonvolatile dissolved organic carbon (DOC) have been determined by high temperature combustion for pore waters from sediments of Middle Valley and Escanaba Trough, Northeastern Pacific, sampled by the Ocean Drilling Program (ODP) Legs 139 and 169, as well as for overlying and near bottom seawater. The DOC values in the interstitial waters are generally greater than those in the overlying water column, ranging from 0.1 to 158 mg of C per liter (mg C/L). Some of the profiles of DOC in the pore waters are similar to total organic C and total inorganic C profiles. DOC maxima at shallow depths of Site 858 are probably enriched due to higher temperature alteration of organic matter. At Sites 856 and 1035, the DOC increases at greather depths due to the hydrothermal activity. The DOC values correlate with MnO and MgO profiles in Hole 856B. The changes below 30 mbsf in the DOC depth profiles of Sites 858 and 1036 parallel those of the volatile hydrocarbon gases. The reference Site 1037 in Escanaba Trough has DOC values that increase with depth according to the increase in thermal stress.